Magnetic bearings are increasingly used in lieu of conventional contact bearings in applications in which lubrication or maintenance of conventional bearings is impractical or otherwise problematic. Since a magnetic bearing supports a rotating shaft by suspending the shaft in a magnetic field, contact between the shaft and the bearing is eliminated during normal operation. In the absence of friction between the shaft and the bearing, bearing and shaft wear is virtually eliminated, effectively extending the life of the bearings well beyond that associated with conventional friction bearings.
Moreover, conventional friction bearings often generate large amounts of heat and/or sparks, particularly in high speed and high output load applications. In applications involving volatile fluids, for example in pumps used in the petro-chemical industry, the use of magnetic bearings greatly reduces the potential for accidental fires and explosions.
Magnetic bearings are particularly useful where access to the bearings is limited, for example in high pressure applications where the shaft and the motor which drives the shaft are enclosed within a sealed chamber. In view of the difficulties associated with disassembling the pump in order to gain access to the bearings, the use of magnetic bearings is highly desirable.
Presently known magnetic bearing systems comprise a plurality of bearings, each having a pair of electromagnets disposed opposite one another, with the rotating shaft interposed therebetween. Each electromagnet includes a magnetic core and a coil wrapped around the core. When voltage is applied across the coil, the current running through the coil induces a magnetic field in the coil which extends beyond the coil and attracts the shaft to the electromagnet. By disposing the electromagnets opposite one another, controlled application of predetermined voltages to the respective electromagnets generates corresponding flux levels at the shaft which tend to urge the shaft along a line extending between the electromagnets in accordance with the applied voltage. Consequently, when the shaft deviates from a desired nominal position between the electromagnets, the voltage at the electromagnetic coils may be varied to create an opposing force on the shaft and thereby return the shaft to its desired position. Moreover, by disposing a plurality of such bearings about the shaft, all degrees of shaft motion may be effectively controlled.
The use of magnetic bearings in the foregoing manner to control the position of a shaft is referred to as levitation. In order to effectively employ magnetic bearings to levitate a shaft, it is necessary that the shaft comprise a ferrous material, e.g. a ferric material, such that the shaft is affected by the magnetic fields created by the electromagnets. Conventional ferric materials may be attracted by electromagnetic fields, but are not generally repelled by conventional electromagnetic fields. Accordingly, it is desirable to pre-bias the opposing electromagnets in each bearing, to permit an increase in the attractive force of one electromagnet and a corresponding decrease in attractive force in the opposing electromagnet. In this way, a predetermined bias (null) force may be established such that the shaft is attracted in diametrically opposite directions by each electromagnet with substantially equal force and, hence, the shaft tends to remain in its natural null position. In practice, it is inevitably necessary to rapidly vary the strength of the flux field to ensure an essentially stable shaft.
More particularly, if the position of the shaft is known or can be inferred, the deviation of the shaft from a nominal position may be determined. Given this deviation, the amount of force necessary to return the shaft to its nominal position may be determined. Presently known systems typically define the amount of force necessary to control shaft position in terms of the voltage or current which must be applied to the electromagnetic coil to effect the desired force on the shaft.
Mature and powerful control algorithms currently exist for maintaining stable levitation through the use of a plurality of magnetic bearing pairs. Known algorithms utilize closed-loop control schemes, wherein a parameter indicative of shaft position (i.e., deviation from a nominal design axis) is used as a feedback parameter in conjunction with a digital, analog, or a combination digital/analog controller to control shaft position.
Control systems have been proposed which measure the position of the shaft directly, for example using an optical or other proximity sensor. Other systems sense shaft position indirectly, for example by monitoring the voltage and/or current present in the electromagnetic coil.
More particularly, a voltage applied across a coil will produce a proportional current in the coil. The current running through the coil, in turn, produces a flux in the gap between the coil and the shaft in proportion to the magnitude of the current. Finally, the force exerted by the electromagnet on the shaft is proportional to the square of the flux produced by the electromagnetic coil. Thus, current-based control systems using coil current as a feedback parameter have been widely proposed in the context of magnetic bearing shaft stabilization systems.
In addition to controlling shaft levitation, magnetic bearings may also be used to reduce cyclic disturbances present in the shaft. In particular, cyclic disturbances are commonly observed in rotating shafts due to, for example, asymmetric mass distribution within the shaft, frame vibration, and load disturbances which are transmitted back through the shaft. These cyclic disturbances, known as harmonic disturbances, typically exhibit frequencies which are integer multiples of the angular frequency of the shaft. For example, a shaft-driven propeller in an outboard boat motor may comprise three propeller blades; the shaft which drives the propeller would thus likely exhibit a harmonic disturbance equal to three times the angular frequency of the propeller shaft.
A magnetic bearing control system is needed which overcomes the shortcomings of the prior art. In particular, a control system is needed which controls the application of levitation and disturbance rejection forces to the shaft more accurately than existing systems. Moreover, a system is needed which coordinates the application of levitation and rejection disturbance forces to the shaft.